This invention relates to multiple access bus communication systems and particularly to systems for reliably and cheaply connecting low cost personal computers.
U.S. Pat. Nos. 4,063,220 and 4,210,780 disclose systems which are considered representative of the present state of the art in multiple access bus communication. U.S. Pat. No. 4,063,220 discloses synchronous baseband bit signalling, and U.S. Pat. No. 4,210,780 describes a carrier modulated system with asynchronous start-stop signalling. Both systems require sophisticated transceiving and adapting equipment to interface between the bus medium and "user devices" served by that medium.
The arrangement shown in the '480 patent requires expensive modem equipment in its transceiver for sustaining its modulated communications, and the schemes shown in both patents require dedicated multi-byte buffers and control circuits which add to the expense of their systems.
Recently, personal computers have become available for serving a market of users having limited financial resources. Such computers may have lower processing speeds than the larger more expensive systems, but they are nevertheless quite versatile and growing in popularity; particularly among individuals and small businesses whose data processing requirements are relatively simple and not dependent on fast handling of large volumes of data.
Such users may acquire several personal computers and eventually have need for interconnecting them for various purposes. For such users, connection to a multiple access bus in the manner described in the above-referenced Metcalfe et al patent would be desirable. It would not require expensive "head end" or master control equipment separate from the user devices, failure at one link node would not disable the entire network, and the baseband signalling method would not require expensive modems. However, synchronous bit signalling itself is not inexpensive. It requires tight timing coordination between the transceiver and the bus, which in turn requires buffering of plural bytes and exclusive dedication of complex hardware for directing real time control operations at the transceiver interface. These functions may involve sufficiently costly adapting equipment to make such connections unfeasible for a large segment of the potential market.
Accordingly, a principal object presently is to provide an inexpensive yet reliable multiple access communication system for connecting low cost computers.
Another object is to provide an inexpensive multiple access bus system for linking user device systems containing inexpensive personal computers, or the like.
Yet another object is to provide for linking such user device systems via passive taps to bus media carrying simple baseband signals, and via primitive and inexpensive apparatus interfacing between the user systems and such media.